The present invention relates to motors in which a rotational movement is generated by means of an ultrasonic piezo-electric effect. More precisely, the invention concerns micromotors of the piezo-electric type comprising two elements intended to effect relative movement to one another and forming respectively a stator and a rotor of the motor, said elements each exhibiting an active surface which, in the course of said relative movement, is placed facing the active surface of the other element, one of such elements being provided with piezo-electric means intended to generate vibratory waves in such element capable of imparting elliptic motions to points on the active surface of such element, such points, in bearing on the active surface of the other element, bringing about such relative movement.
Such a motor is described in an article appearing in the review "Techno Japan" Vol. 19-No. 5, May 1986. This motor includes a stator in the general form of a disc and consisting of two flat connected elements, one being in stainless steel and the other of piezo-electric material. On the visible face of the stainless steel element is arranged a ring, a series of small blocks separated from one another and concentric to the axis of the stator disc. The rotor of this motor also exhibits the form of a disc which is rotatably mounted coaxially to the stator in a manner such that one of its faces rests on the free face of the stator blocks. Such rotor, which is also formed in stainless steel, includes in the zone facing the stator blocks a ring of plastic material offering a high coefficient of friction.
When the piezo-electric element of the stator is excited, it produces a progressive wave in the stator blocks which is propagated in a circular manner along the blocks so that each of the points of the blocks facing the rotor effects a very small axial motion combined with an annular motion according to a composite elliptic motion which, applied to the friction ring of the rotor, causes the latter to advance in rotation around the axis of the motor.
Given that the amplitude of motion of the points of the stator blocks is extremely small, in order that a perceptible power may be taken off the axis of the rotor, such amplitude should be greater than the irregularities in the surface of the blocks which face the rotor. Now, such amplitude is on the order of a micron or some microns so that it is necessary to machine such surfaces with an extreme precision.
Another disadvantage of this motor is that it is necessary to employ two pairs of excitation electrodes for the piezo-electric element energized by out-of-phase electric potentials, otherwise it is not possible to generate a progressive wave in the stator. This requirement complicates the energization circuit of the motor and thus increases the cost thereof.
Finally, the known motor is not very readily adapted to the transmission of low value couples as would be desirable in horological applications for instance since, under these conditions, the yield is mediocre.
Another construction of a piezo-electric micromotor is described in the journal IEEE, 1989, pages 735 to 738, by T. Takano, Y. Tomikawa, T. Ogosawara and H. Hirata. The article in question entitled: "Construction and characteristics of ultrasonic motors using piezo-ceramic annular plates", describes a motor including a first element exhibiting a generally planar form and associated with means intended to generate therein vibratory waves at an ultrasonic frequency enabling the imparting of elliptical motions to active peripheral points of such elements within its plane.
Each active point is in engagement with a motion transmission surface of a second element of annular form placed around the first element in order to bring about a movement of the second element relative to the first element by virtue of said elliptical motion.
This known construction, although it is capable of transmitting a power greater than that which it is possible to transmit in the construction discussed hereinabove, also presents the disadvantage of necessitating an extreme precision in the outer surface of the vibrating element and of the profile of the active surface of the annular element, the two elements having to be adjusted so that there exist practically no play between the two, while nevertheless permitting a relative motion between them.
Furthermore, such motors have the same disadvantages as the motor of the prior art discussed hereinabove, namely they require a two-phase energization voltage and exhibit low yield when they are conceived for the transmission of low couples.
The invention has as its purpose to provide a piezo-electric motor of the general type described hereinabove, but avoiding the cited disadvantages.